Conventional methods for making printing plates so far known in the art include such systems as shown in FIGS. 17a-17b, 18 or 19a-19c.
The plate-making system shown in FIG. 17a-17b is generally implemented as follows. In the first place, an aluminium plate 100 is provided, which has been polished by such polishing techniques as ball or brush polishing or "grained" in the jargon of the field, and a photosensitive resin layer 101 is formed on this aluminium plate 100 to form a printing substrate. Then, a plate-making film 102 is located in opposition to the resin layer 101, followed by pattern exposure (FIG. 17a) with ultraviolet rays 103, development and drying. In this way, a printing plate including a printing area formed by the resin layer 101 is produced, as shown in FIG. 17b. In this regard, it goes without saying that the film 102 has been subjected to page make-up, as with a film output from color scanners now referred to as layout scanners or page make-up scanners. This is also true of the description that follows. For color printing, it is a matter of course that printing plates must be prepared in association with four colors i.e., yellow (Y), magenta (M), cyan (C) and black (K), respectively, and four such printing plates for Y, M, C and K may be prepared by using a film for each color instead of the film 102 of FIG. 17a.
Printing plates comprising grained aluminium plates and photosensitive layers formed thereon are generally called PS (presensitized) plates and are now commercially sold on the market. The PS plates are costly, but serve well due to the presence of the pre-coated photosensitive layers.
In print shops having photosensitive layer coating equipment, photosensitive layers have been actually coated on such grained aluminium plates as mentioned above. The thus prepared plates, called wipe-on plates, are less costly and more sensitive than the PS plates, but are inferior in serviceability to the PS plates due to some coating steps being needed. Serious limitation is imposed on the operation of the wipe-on plates as well, because they have such a short pot-life or the time span from their being coated to their use, that they must be exposed to light just after the formation of photosensitive layers thereon.
This is the reason that the PS plates are now virtually supplanting the wipe-on plates. As already mentioned, the PS plates can serve well, but the sensitivity of their photosensitive layers is not good enough, because they should stand up to long-term storage with the photosensitive layers coated on them. Generally speaking, the higher the sensitivity of the PS plates, the more they are reactive with respect to heat, thus often resulting in their fogging due to thermal reactions during storage. This makes it very difficult to increase the sensitivity of the PS plates.
Referring to FIG. 18, there is shown a printing substrate obtained by forming a photosensitive resin layer 106 on a grained aluminium plate 105 and forming thereon a layer 107 comprising a silver emulsion. This substrate may be processed into a printing plate by similar pattern exposure, development and drying as described in connection with FIGS. 17a-17b. This printing plate has been developed with a view of making up for the defect--low sensitivity--of the PS plates. More exactly, a silver emulsion layer is formed on a PS plate, which is in turn subjected to primary, low-energy exposure, while making use of the high sensitivity of the silver emulsion, thereby developing the silver emulsion. Then, the resulting blackened silver particle pattern is used as the original for all over uniform exposure (secondary exposure) and then development, thereby obtaining a printing plate. The objective is to take advantage of such low-energy exposure as laser-scanning exposure or projecting exposure.
Laser-scanning exposure of printing plates is a technique of vital importance especially when printing is to be carried out in printing plants located at remote places with information fed through communications lines, as is the case with preparing printing plates for "The Wall Street Journal". Projecting exposure, on the other hand, enables printing plates to be immediately prepared, if only reflection copies are available, and so can dispense with such time consuming steps of making film copies through process cameras as required conventionally.
These have a great merit of making low-energy exposure possible, but are costlier than the PS plates because expensive silver emulsions have been laminated thereon--to say nothing of it.
The plate-making process as shown in FIGS. 19a-19c resorts to one electrophotographic technique, wherein a photosensitive material 110 comprising a photoconductive material is first electrostatically charged by corona discharge in a uniform manner, then pattern exposed to light 112 having a given wavelength through a film 111 (FIG. 19a), and finally coated with a toner 113 (FIG. 19b), whereby the toner 113 is deposited onto only a portion of the material 110 that has not been exposed to the light 112. After that, this material is transferred and fixed onto a grained aluminium plate 114, thereby obtaining a printing plate including a printing area demarcated by the toner 113 (FIG. 19c).
Referring to FIGS. 20a-20c, there is shown a plate-making process relying upon another electrophotographic technique, wherein a photosensitive material 123 comprising a photoconductive material layer 122 and a grained aluminium plate 121 is first electrostatically charged by corona discharge in a uniform fashion, then pattern exposed to light 125 having a given wavelength through a film 124 (FIG. 20a) and finally coated with a toner 126, whereby the toner 126 is deposited onto a portion of the material 123 that has not been exposed to the light 125. After that, the toner 126 is fixed in place (FIG. 20b) and the exposed region of the photoconductive material layer 122 is etched out using the toner 126 as a resist, thereby exposing portions of the grained aluminium plate 121 to view (FIG. 20c). In this way, it is possible to obtain a printing plate in which the rest of the photoconductive material layer 122 and the toner 126 define a printing area and the exposed region of the grained aluminium plate 121 demarcates a non-image area.
As described above, various plate-making processes have been known in the art, but they have involved the following problems. That is to say, the plate-making process shown in FIGS. 17a-17b should use a highly sensitive type of resin, because it resorts to exposure to ultraviolet rays. In general, however, a class of material highly sensitive to ultraviolet rays is so poor in thermal stability that it is likely to suffer the so-called "thermal fogging". The highly sensitive type of resin, on the other hand, has a molecular weight so low that it offers a problem in connection with the resistance to printing required, i.e., the mechanical strength that printing plates are required to have. Thus, considerable difficulty will be encountered in finding a type of material that is satisfactory in terms of both sensitivity and resistance to printing.
By contrast, the printing substrate shown in FIG. 18 can be made more sensitive by the use of silver emulsions and can use conventional types of resin for the resin layer 106. However, this has a serious defect of being costly.
In the process shown in FIGS. 19a-19c, it is essentially required that the transfer of the toner image formed on the photosensitive material 110 onto the aluminium plate 114 occur at a relative speed of zero. When this relative speed deviates from zero, misalignment or pattern distortion occurs during printing. This is even so especially when printing plates of a large area are used. Also, the toner is likely to fall into disarray, triggering a serious drop of resolving power.
The process shown in FIGS. 20a-20c, without recourse to toner transfer, is more unlikely to cause the toner image to be disarrayed, as compared with the process shown in FIGS. 19a-19c. Since the photoconductive material layer forms part of the printing area, however, it must be satisfactory in terms of both sensitivity and mechanical strength.
Generally, photoconductive material layers are obtained by dispersing such photoconductive pigments as zinc oxide in polymeric materials. In order to achieve sufficient sensitivity, however, they should contain zinc oxide in so large an amount, say 80% in weight ratio, that they become fragile and lack in resistance to printing.
In order to solve these problems, it has been proposed to use a process wherein the surface, exposed to light, of the photoconductive material layer 122 is made hydrophilic as by phosphoric acid without etching, while the toner is fixed on such a photosensitive plate 123 as shown in FIG. 20b, whereby the printing area is defined by the toner region and the non-image area is demarcated by the exposed region of the photoconductive material layer thus made hydrophilic. According to this process, however, what has been made hydrophilic is only the photoconductive pigment, e.g. zinc oxide. This, combined with the fact that the polymeric material serves as a binder, renders it impossible to afford sufficient hydrophilic nature to the photoconductive material layer, thus often causing such an accident as scumming. Once ink has been deposited onto the non-image area, there is no choice but to replace the plate with a new one.
With the electrophotographic systems used so far and described above, if it is intended to use the photoconductive material layer as a printing area, then photoconductivity and the mechanical strength required for the printing area are incompatible with each other. On the contrary, if it is intended to use the photoconductive material layer as a non-image area, then the hydrophilic nature required for the non-image area is far from satisfactory. This may be solved by transferring the toner on other grained aluminium without being fixed on the photosensitive material. However, there arises another problem that the toner image falls into disarray, rendering it impossible to maintain resolving power.
In the foregoing, the conventional printing plates, conventional processes for making printing plates and problems in association therewith have been described. In the description that follows, reference will be made to conventional page make-up systems.
In the prior art page make-up systems, originals, if limited in number, are individually applied on the input drums of associated color scanners, and are then separated into four colors C, M, Y and K under the preset color separation conditions and at the prescribed magnification, followed by electronic page make-up operation with page make-up equipment. When a large number of originals are used, on the other hand, the duplication of the color originals is made at an intermediate duplication magnification found by: Intermediate dublication magnification =
(Final magnification)/(scanning magnification). This enables the originals to be separated into colors by a single cycle of scanning at a plurality of different magnifications. Thus, the scanning magnifications can be standarized by making duplicates of the color originals at an intermediate step. With these duplicates, the originals having different final magnifications are mounted on the same scanning drum, thereby pushing on effective color separation.
In an effort to rationalize page make-up steps, some intermediate duplicates of the originals are mounted on the scanner drum, while taking the final magnification into account.
These procedures are now called the duplication assembly.
However, the former method is very troublesome and time consuming, since it is required to preset the color scanner's separation conditions for each of the individual originals in terms of input magnification and color separation. Such presetting of color separation conditions is largely dependent upon the experience of operators and is very exacting. Taken altogether, this method has failed to boost the efficiency of operation of costly color scanners. In addition, computer operation is required for page-making-up large volumes of data input for each original according to layout instructions, and this is a time consuming process. As a result, there have been drops in the efficiency of operation of not only color scanners but computers for page-making-up as well.
A major problem with the latter "duplication assembly" is that it takes much time and expense to make duplicates, although the presetting of color scanner's separation conditions is achievable in a single operation.
Other incidental problems are that duplication makes images degrade in quality, e.g. renders image quality hard in tone; a plurality of originals undergo color separation under the same setting-up conditions--because the cycle of scanning is one, so that color separation may not always occur under the optimum setting-up conditions for the individual originals; and for similar reasons, this procedure cannot be used for originals differing largely in their setting-up conditions.
Having been accomplished against such a background, the present invention seeks to provide a printing plate that is well resistant to printing but dispenses with any transfer process by forming a toner image directly on a charge carrier medium and a method for making it. Another object of this invention is to provide a page make-up system using a charge carrier medium, wherein page-making-up is performed directly on a charge carrier medium by exposure with the application of voltage, thereby boosting the efficiency of page make-up operation.